Photoinduced Charge Separation and Charge Recombination to a Triplet State in a Carotene−Porphyrin−Fullerene Triad

A molecular triad consisting of a diarylporphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene (C60) has been prepared and studied using time-resolved spectroscopic methods. In 2-methyltetrahydrofuran solution, the triad undergoes photoinduced electron transfer to yield C−P•+−C60•-, which evolves into C•+−P−C60•- with an overall quantum yield of 0.14. This state decays by charge recombination to yield the carotenoid triplet state with a time constant of 170 ns. Even in a glass at 77 K, C•+−P−C60•- is formed with a quantum yield of ∼0.10 and again decays mainly by charge recombination to give 3C−P−C60. The fullerene triplet, formed through normal intersystem crossing, is also observed at 77 K. The generation in the triad of a long-lived charge separated state by photoinduced electron transfer, the low-temperature electron transfer behavior, and the formation of a triplet state by charge recombination are phenomena previously observed mostly in photosynthetic reaction centers.